While the recent trend is toward higher integration and size enlargement of microelectronic devices, there is a demand for resin-encapsulated packages of thin profile and reduced size. It is accordingly required that the surface protective layer and interlayer dielectric film in semiconductor devices and the re-distribution layer of semiconductor packages be formed of materials having better electric properties, heat resistance and mechanical properties. Polyimide resins are one class of materials that meet the required properties. For example, polyimide resins can be modified to be photosensitive. Attempts are made to use such photosensitive polyimide resins, because the pattern forming process can be simplified and the complex manufacture process can be shortened. See Patent Documents 1 and 2.
A film of polyimide resin is generally prepared by reacting tetracarboxylic dianhydride with diamine to form a polyimide precursor (or polyamic acid), applying a solution or varnish of the polyimide precursor to form a thin coating such as by spin coating, and causing thermal cyclo-dehydration or ring-closing reaction. Through this cyclo-dehydration step, the polyimide resin is cured. In the case of polyimide resins resulting from polyimide precursors, however, a problem arises that volume shrinkage attributable to dehydration or imidization can occur upon curing, leading to a loss of film thickness and a lowering of dimensional accuracy. Nowadays, it is desired to form a film at low temperature. What is needed in this sense is a polyimide resin which can be subjected to cyclo-dehydration at low temperature to form a film having physical properties comparable to those of the film obtained from cyclo-dehydration at high temperature. Nevertheless, when the polyimide precursor is cured at low temperature, the cured film becomes brittle or degraded in physical properties because of incomplete imidization.
On the other hand, studies are made on photosensitive resins based on other heat resistant polymers which do not need cyclo-dehydration unlike the polyimide precursors. See Patent Documents 3 to 6. Particularly in such applications as the re-distribution layer in semiconductor packages, a positive photosensitive resin composition which is developable in alkaline aqueous solution and yet can form a pattern having high heat resistance is needed from the aspect of environmental load reduction.
Although the positive photosensitive resin composition which is developable in alkaline aqueous solution has some acceptable properties including heat resistance, further improvements in sensitivity and resolution are needed.